Not only has Rehnquist not been writing opinions, he hasn't even been voting in cases where the other Justices aren't 4-4. He also hasn't been attending oral argument. If they need him to break a tie, he will read the briefs and listen to the tapes of the argument.
Off the top of my head, I can't remember a 5-4 where he's been in the majority, so I suspect that the 4-4s are being held up not only because they're likely to be the hardest cases and would take longer to write anyway, but also because the Chief isn't up to working at anything like a normal pace.
I've gotten accustomed to all of the opinions this Term saying: "Justice X delivered the opinion of the Court, in which Justices A, B, C, D, and E joined. Justice Y filed an opinion concurring in the judgment. Justice Z filed a dissenting opinion. The Chief Justice took no part in this case."
So it was a bit odd to see his name atop an opinion that all 8 of his colleagues joined. Clearly, his vote wasn't needed. Maybe he's feeling a bit better? Whatever the case--and I mean this sincerely--it's good to have him involved. Having the Chief uninvolved in case after case is not a tenable situation, and it's good for the institution to have him working on cases until he retires.